Multi-layer circuit boards include boards such as PUBS (printed circuit board), PWBs (printed wiring boards), and flex circuits. Boards that are etched rather than printed are also included with the definition of multi-layer circuit boards.
Multi-layer circuit boards are made up of layers of semi-transparent insulating material such as FR-4 or Getek, having circuitry processed thereon. Generally the circuitry is copper although other conducting material can be used.
Circuit patterns are generated on conductive cladded sheets of transparent insulating material. The sheets are then stacked, aligned and bonded to create a multi-layer circuit board.
A problem that can occur with circuit board fabrication is that the conductive layers are processed out of sequence than what was designed. For instance, a circuit board comprised of 6 conductive layers could have a required stacking sequence of 1 2 3 4 5 6. A fabricator could mistakenly place the order as 1 4 5 3 2 6. Even though the layers are out of order as designed, the board might generally perform most, if not all, of the functions the circuitry was designed for. This is due to the fact that connection points among all the layers are generally designed at the same point on the board. Although all electrical tests would test the circuit board as GOOD, in fact the rearrangement could cause unusual characteristics that would be undetectable to typical test equipment.
An example of affects on the circuit board not readily detected by typical test equipment is a dedicated internal Ground plane which must be adjacent to a dedicated internal Power plane. This creates a much needed capacitance within the circuit board. Switching the layers would remove the Ground plane away from the Power plane eliminating the capacitance. Another example is the creation of a radio frequency transmission or strip line. In these examples, a copper pattern on one layer requires a specific distance to another Ground plane layer. Changing the placement of the layers changes the distance needed for the proper frequency in the radio transmission or in the strip line.
If the placement of the layers is wrong, the result of the circuit boards of the two above examples would be that the circuit boards perform outside their designed specifications. These types of requirements are very difficult to test for. Accordingly, testing for this type of performance is not done and is assumed to be correct if other testing proves within design parameters.
A finished circuit board normally looks "correct" whether the internal conductive layers are placed properly or not. Visual inspection of such boards to detect misplacement requires a very trained eye for specific circuit details which would probably not be available to the board fabrication house, inspectors of the boards, or factory staff. Accordingly, there is a need to provide a simplified method of allowing fabricators and factory inspectors to determine visually, through some type of layer inspection window (sometimes referred to as a coupon), if the layers have been placed properly.
Attempts have been made to provide a means of visually inspecting the arrangement of the layers through placing various patterns on the layers which can be seen in a layer window. Since the insulating material is transparent, numbers or shapes have been added to each layer for inspection of the construction through depth comparison. Unfortunately, the eye cannot easily distinguish the depth of shapes or images through the transparent layers. As the layers deepen, the difference in depth of the image(s) becomes difficult if not impossible to determine.
Accordingly, a means to visually detect quickly whether the layers of the printed circuit board are properly placed as designed is needed.